This invention relates to a method for making semiconductor die, wherein at least two dielectrically isolated islands are created in each of the die and in each of these islands an epitaxial layer is formed having a uniform and predetermined thickness from island to island and from die to die.
Numerous processes have been used to make semiconductor die with dielectrically isolated islands or pockets. Such die are employed in the manufacture of integrated circuits wherein separate transistors are formed in separate of the isolated pockets.
In one such method a system of grooves is formed in a major surface of a silicon wafer being heavily doped with N-type impurities. A silicon dioxide layer is grown over that wafer surface and the surface of the grooves, and a thick self-supporting polysilicon layer is deposited over the silicon dioxide layer. Subsequently, a major portion of the N+ silicon wafer material is mechanically removed by lapping to leave a plurality of shallow N+ islands separated from each other by the aforementioned silicon dioxide layer. A lightly N+ doped silicon is then epitaxially grown over the thin N+ islands. In that step there are simultaneously formed lightly N-doped polysilicon strips over the initially exposed silicon dioxide that is at the bottom of the grooves. This doped polysilicon is not an excellent insulator and is removed by a selective etching step, the resulting holes are filled and the fill material and epitaxial layer patches over the N+ islands are planarized by lapping. These later steps for obtaining good isolation between pockets by removing polysilicon strips, filling and lapping is complex and costly but the last step of lapping is of particular concern because lapping tends to remove some of the epitaxial layer that has an inherently uniform thickness. Lapping is a relatively crude process for removing material and the uniformity of thickness of the as-deposited epitaxial layer patches is thereby degraded.
In another prior art process a lightly N-doped epitaxial layer is grown over a heavily N-doped silicon wafer. A matrix of grooves is formed completely through the epitaxial layer and slightly through the N/N+ interface. Silicon dioxide is grown over the exposed epitaxial material and thereover is deposited a first thick self-supporting layer of polysilicon. The exposed N+ wafer surface is then lapped away to leave thin N+ patches separated by the SiO.sub.2 in the matrix of grooves. The above-noted matrix of grooves separates blocks of the N and N+ silicon layers from each other. An insulating layer is formed over the N+ silicon patches.
Now, in each of the patches of this N+ material a second system of grooves is etched completely through the N+ layer and completely through the N epitaxial layer to divide each P+ patch and associated epitaxial patch into islands bounded by some grooves from both the first and second sets of grooves. A second silicon dioxide layer is grown over the surface of the thin N+ islands and surfaces of the second sets of grooves.
A second thick self-supporting layer of polysilicon is formed over the N+ silicon patches. The first thick self-supporting layer of polysilicon is now removed by lapping until the second silicon dioxide layer covering the epitaxial material is laid bare. Again, this process is complex and costly, and more importantly, uses the crude mechanical lapping process to expose but in this case hopefully not penetrate the second silicon dioxide layer which is difficult to control especially over the entire surface of a wafer-sized workpiece.
It is therefore an object of this invention to overcome the above-noted shortcomings of the prior art.
It is also an object of this invention to provide a noncritical method for making an integrated circuit die having dielectrically isolated islands comprised of a uniformly thick epitaxially grown layer.
It is a further object of this invention to provide a method for making a plurality of die with dielectrically isolated islands of semiconductor material wherein an outer portion of each island is a very thin epitaxial layer of highly uniform thickness from island to island and from die to die.